The production of inorganic silicate materials by the thermal expansion of hydrated silicates has been disclosed in the prior art. Many of the known methods utilize the liquid content of the silicate composition as a blowing agent to produce a cellular structure. Such foamed masses exhibit good thermal and acoustical insulating properties and have been used in a variety of construction materials. Wallboard has been prepared, for example, by the formation of a plastic silicate-containing composition between paper liners, followed by thermal expansion of the silicate. In addition, granules of silicate have been expanded at high temperatures to form "popcorn" materials useful in insulating applications.
One difficulty with many of these silicate-based materials is their deterioration upon exposure to water for extended periods of time. In an attempt to overcome this water solubility, processes have been developed wherein various insolubilizers are added to the silicate composition. Two such processes are described in U.S. Pat. No. 3,719,510 and U.S. Pat. No. 3,765,919. These patents disclose the preparation of expanded silicate aggregates by mixing aqueous alkali metal silicate with anhydrous alkali metal silicate, dispersing insolubilizer additives into the liquid, curing the fluid composition, drying, then grinding and thermally expanding the material.
Although the aggregate products obtained by these methods have good physical properties, the processes involve several steps which are difficult to control. Liquid alkali metal silicate must initially be mixed with anhydrous silicate in precisely the correct ratio to eventually produce a properly hydrated silicate. The insolubilizer additives must then be admixed before the viscosity of the liquid composition increases to the point that uniform dispersion of the additives becomes impossible. The resulting viscous plastic composition must then be rapidly transferred from the mixer to the curing operation in order to prevent separation of solids and to avoid a difficult removal problem should the mixture cure too rapidly. Heat is also required throughout the mixing process in order to maintain the composition in the desired liquid or plastic state prior to curing.
In addition, the insolubilizer additives may be dusty and difficult to handle and increase the cost of the aggregate product. They also tend to increase the bulk density of the aggregate.
It would, therefore, be desirable to provide a process which does not require the handling and measurement of liquid silicates having varying ratios of alkali metal oxide to silicon dioxide. In addition, it would be an advantage to provide a method in which the silicate composition remains in an essentially dry state until the curing stage, eliminating the problems associated with drying operations and the handling and mixing of viscous masses. A further advantage would reside in the reduction or complete elimination of insolubilizer additives.